Polymer for spring desing

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Polymer for spring desing

Polymer for spring desing

I'm looking for some technical advice on a design which I working on. I'm looking for a type of plastic that works for a small spring (Free length of 10 mm and 5 actives coil) with low compression resistance. I'm aware that the plastic must have a higher Young's modulus in order to prevent a permanent deformation.

In the picture below you see the model spring that I need, but obviously in plastic.

RE: Polymer for spring desing

The comment about using a higher modulus is based on the idea that it needs to be initially stiffer to still have usable stiffness at the projected end of life.

Pud is dead on. I fought this at a previous employer. They always wanted to mold in a plastic spring, they kept trying it and it never worked, I kept explaining why, and they would get it for about 15 minutes, then next week they were right back at it. Get Trantina and Nimmers book, Structural analysis of thermoplastic Components and check out the section covering isochronus stress strain curves. Your E goes way down,and the stiffness and performance will change dramatically over time.

RE: Polymer for spring desing

OK, there's really no need to reinvent anything. Polymer spring-like objects already exist. This came from a spiral notebook; it's stiffer than a polymer Slinky, and obviously is holding its shape reasonably well.

RE: Polymer for spring desing

The notebook binder is not strained. Indeed it is spring 'like' also. It's not functioning as a spring.
This is my last comment on this thread. If the op wants a plastic spring, that's fine. Good luck.

RE: Polymer for spring desing

I think the first non-metallic Vette springs were pultruded glass filled,if memory serves (often it doesn't lately). I think later versions are carbon fiber. I did see an articular about a non-metallic coil spring on some cost-no-object supercar (maybe the new Ford GT). They were horribly expensive.

Rick Fischer
Principal Engineer
Argonne National Laboratory

RE: Polymer for spring desing

I may have seen such a spring used either on an oversized ball point pen as novelty item or on a child toy once but never after. I have seen plastic slinkies for kids and perhaps that material would be appropriate. If you know about the steps used in spring design, see if you can get a catalog such as "Noryl thermoplastic resins-Design" which will list the mechanical properties of their plastics. One property of interest is the shear modulus of elasticity, of what magnitude for your project, I do not know but my gut reaction would the closer to steel value (11.5x10^5) the better. Summary of the steps involved in spring design are readily available in ME handbooks.

RE: Polymer for spring desing

I think some of you you are missing an important point. Yes plastic springs are possible, but not very practical if they must endure a sustained load. The OP never tells us how the spring is loaded, or what the product is, or it's expected life. But his comment about a higher modulus suggests he's aware of isocronous stress strain curves and their use in designing for a sustained load. Plastic creeps under sustained load at room temperature. More correctly in this case, if the spring is loaded to a constant deflection, the load will decrease, which is stress relaxation. A polymer is a living thing. The molecules are vibrating, trying to get to their lowest energy state. There are conformational changes, one molecule moving with repsect to another. These happen at a very slow rate because the molecules must overcome energy barriers for the changes to take place, and randomly in all directions, so the polymer does not change shape. But if the polymer is loaded, the rate of these conformational changes increases in proportion to the load intensity, and the energy barriers are decreased in the direction of loading and increased in the opposite direction, ie no longer random in all dimensions. The result is a change in shape. Apply a small load for a long time and you get creep. Apply a high load for a short period of time, and you get yield. It is essentially the same process played out on different time scales. You can change the rate by adding fillers or reinforcing fibers, or crosslinking the polymer chains, or increasing the degree of crystallinity, or choosing a polymer with more aggressive inter molecular bonds. But it still creeps. So a slinky that sits in a toybox unloaded until a kid pulls it out to play with it works fine. And a plastic leaf spring that is more carbon fiber than plastic works fine. BTW, the plastic in the spring is not a thermoplastic, but more correctly a thermoset with a very high cross-link density, which largely prohibits the conformational changes mentioned above.

Rick Fischer
Principal Engineer
Argonne National Laboratory

RE: Polymer for spring desing

You might be able to mold something spring-like with a much larger cross-section, and probably fewer turns, but the mold will be expensive and the yield will be uncertain.
... and it will still be plastic, meaning it will deform plastically as long as a load is present.

You could machine a spring-like thing of constant pitch, by machining through plastic tubing on a mandrel, again thick-walled tubing, and it will still be plastic. My favorite modelmaker made a triplex polypropylene spring in this way for a prototype check valve. I didn't pursue it because it was protected by patents at the time. (See Dab-O-Matic or similar ISTR.)

Triplex here means like a three-start thread with a high helix angle. Note that the high helix angle also means substantial rotation as the spring is compressed; it's not clear if that would be a problem for your intentions.

You could form a metallic spring similar to your photo and have it coated with plastic, e.g. some kinds of 'teflon', as is used for coating o-rings for refrigeration. I think you'd need to leave the end coils open to get full coverage with few flaws.

How big is your R&D budget?

Mike Halloran
Pembroke Pines, FL, USA

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